I just finished building this using the layout on page 2. I added an input cap and a pull down resistor as discussed on page 3. I picked through 10 jfets and got 3 that bias at 5.3-5.6V, which I guess is a little high, but I kept it as is because it sounds good. Not as much gain as I thought it would be, but it sounds good without any tweaks.

I want to sort some J201s on voltage without having to open up my Plexi-Drive again (it was a tight fit with the veroboard). Is there any way to create a separate voltage test circuit for j201s? I can't be that hard but I'm a noob so I can't, for the life of me, figure out how. Thanks!

If you want to see how they're going to behave (voltage-wise) in the Plexi-drive circuit, just build a little circuit mimicing one of the gain stages (since they're all biased the same). Put a transistor socket on your board with a 1k resistor from source to ground, a 1M resistor from gate to ground, and a 15k resistor from drain to your 9v power source. Voila. Measure your voltages - no mathematical calculations needed to see how they're going to perform in a Plexi-drive configuration.

BAARON wrote:If you want to see how they're going to behave (voltage-wise) in the Plexi-drive circuit, just build a little circuit mimicing one of the gain stages (since they're all biased the same). Put a transistor socket on your board with a 1k resistor from source to ground, a 1M resistor from gate to ground, and a 15k resistor from drain to your 9v power source. Voila. Measure your voltages - no mathematical calculations needed to see how they're going to perform in a Plexi-drive configuration.

Thanks for the confirmation, I build exactly that circuit just before your reply and was able to measure the voltages These J201s are all over the place, out of 10 only 2 or 3 where within the 4.5 - 5.0V range. Gonna try it with these new ones, the old ones had voltages of 5.8 and higher!

I built a box with two Plexi-Drives using the layout Mark made (http://www.tagboardeffects.com). I already built a single one but I loved the tone so much that I gave that one to my friend and built a double one for myself. On the right side I keep it low-gain for "tube-like" saturation/crunch and the left side I have a bit more gain for some rhythm stuff. I also use Marks offboard wiring layout which eliminates popping on every pedal because the input is grounded through the switch.

Anyways, here's a photo of my build:

I put the two bass switches on the back.

Sorry for the large image. I just couldn't be bothered making it any smaller.

I recently stumbled on the Plexi Drive while searching for information about j201 pedals and designs. So I decided to breadboard this one

I really like the sound it gives but I have some doubts and questions about all the gain/biasing stuff. When I ordered parts for this build I didn't know JFETs and especially j201's had to be biased, so I only got a bag of 4 j201's... At first the sound was gatted because my drain voltages were sitting at approx. 2.6V for each transistor with 15k stock resistor (they all seemed to have the same characteristics - except for Q3...). So I had to set the drain voltages for Q1, Q2 and Q3 by modifying the stock 15k resistors with these values : 10k for Q1 and Q2, and 7.4k for Q3. This gives me approx. 4.8V as drain voltage for all transistors.

However, I have 2 questions that puzzle me :

- when the gain control is turned all the way down, I have no sound : is this the normal behaviour of the design ? I know the 1k resistor going to ground on the first gain stage should ensure a minimal gain, but just wondering...- how much can the overall gain be affected by using lower values for drain resistors ? Is it correct to assume the gain for each transistor is roughly calculated by dividing the drain resistor by the source resistor value ?

When the gain is cranked all the way up the sound is overdriven but it seems a little anemic to me...

dludo59 wrote:I recently stumbled on the Plexi Drive while searching for information about j201 pedals and designs. So I decided to breadboard this one

I really like the sound it gives but I have some doubts and questions about all the gain/biasing stuff. When I ordered parts for this build I didn't know JFETs and especially j201's had to be biased, so I only got a bag of 4 j201's... At first the sound was gatted because my drain voltages were sitting at approx. 2.6V for each transistor with 15k stock resistor (they all seemed to have the same characteristics - except for Q3...). So I had to set the drain voltages for Q1, Q2 and Q3 by modifying the stock 15k resistors with these values : 10k for Q1 and Q2, and 7.4k for Q3. This gives me approx. 4.8V as drain voltage for all transistors.

However, I have 2 questions that puzzle me :

- when the gain control is turned all the way down, I have no sound : is this the normal behaviour of the design ? I know the 1k resistor going to ground on the first gain stage should ensure a minimal gain, but just wondering...- how much can the overall gain be affected by using lower values for drain resistors ? Is it correct to assume the gain for each transistor is roughly calculated by dividing the drain resistor by the source resistor value ?

When the gain is cranked all the way up the sound is overdriven but it seems a little anemic to me...

Thanks guys !

That is wrong.... Brian recommend set up all j201 (matched) to 4.5V - 5VDC max in drain voltage., do not change any resistor value.. You must sort a set of 3 J201 to accomplish it. This is important for to have a true clone PlexyDrive..

Gboutique wrote:That is wrong.... Brian recommend set up all j201 (matched) to 4.5V - 5VDC max in drain voltage., do not change any resistor value.. You must sort a set of 3 J201 to accomplish it. This is important for to have a true clone PlexyDrive..

Ok, guess I'll have to buy a bunch of j201 and pray to have the perfect trio then !

Gboutique wrote:That is wrong.... Brian recommend set up all j201 (matched) to 4.5V - 5VDC max in drain voltage., do not change any resistor value.. You must sort a set of 3 J201 to accomplish it. This is important for to have a true clone PlexyDrive..

Ok, guess I'll have to buy a bunch of j201 and pray to have the perfect trio then !

You have to match 3 j201 at 4.5-5v (drain voltage). Your only luck is to buy several j201 and hope to stumble on a good trio. Unfortunately you have no way to predict the values you will get, so you have to buy large quantities. Check Ebay for that.